1. Field of the Invention
This invention relates to a multiple-displacement engine in which operating state of the engine can be changed between a full-displacement state in which all the cylinders output power and a part-displacement state in which a part of the cylinders are cut out or are made inoperative according to the operating condition of the engine.
2. Description of the Prior Art
In order to improve fuel economy, there has been proposed a multiple-displacement engine which can be operated either in a full-displacement state or in a part-displacement state according to the operating condition of the engine. Generally in engines of the kind, fuel supply to a part of the cylinders is cut to make inoperative the cylinders (the part-displacement state) when the engine is operated at a low speed under light load. Accordingly, the engine is generally operated in the part-displacement state when it is idling.
Even when the engine is operated in the full-displacement state, the engine vibrates stronger when the engine is idling than in other operating regions. When the engine is idling in the part-displacement operation of the engine, the engine vibrates further strong. The solid line in FIG. 1 shows the vibration of the engine when the four-cylinder engine is idling in the full-displacement state (all the four cylinders are operative), while the dotted line shows the vibration of the engine when the same engine is idling in the half-displacement state (only two cylinders are operative). As can be seen from FIG. 1, when the engine is idling in the half-displacement state, the number of peaks of the vibration is reduced to the half of that when the engine is idling in the full-displacement state. On the other hand, the amplitude of torque fluctuation is substantially enlarged when the engine is idling in the half-displacement state. Accordingly, in the multiple-displacement engine, there is a problem that the driving comfort is lowered during idling.